In the case of chokes of an EMC filter through which a direct current flows in addition to the interference that is to be suppressed, it is known to prevent the direct current from saturating the core of said EMC filter by virtue of the fact that the chokes in a forwards conductor and an associated rearwards conductor for the direct current are wound onto a common core in such a manner that the magnetizations of the core due to the direct current flowing through the two chokes are eliminated. Common mode interference that is coupled in on one side of the chokes into the forwards conductor and the return conductor results in contrast in the core being magnetized in a variable manner starting from zero and is accordingly damped. Chokes of this type having multiple choke windings on a common core are described collectively as current-compensated chokes.
It is known for example from the products Sunny Boy 4200 TL HC and Sunny Boy 5000 TL HC of the applicant to use in each case a current compensated choke for the two lines that lead from one of multiple inputs of a multi-string inverter to the associated DC/DC converter. However, the EMC filter of a multi-string inverter of this type having a current-compensated choke for each input comprises in its entirety on the one hand a considerable amount of mass due to its chokes and at the same time its filtering effect is not optimal across the different operating modes of the multi-string inverter.
The hitherto typical operating mode of a multi-string inverter is characterized by a mutually independent MPP tracking of the individual photovoltaic generators that are connected to the multi-string inverter. However, a further, hitherto non-relevant operating mode also comes into focus with respect to a use of multi-string inverters within larger photovoltaic systems. This operating mode is characterized by a hard parallel connection of at least two DC inputs of the multi-string inverter. The photovoltaic generators that are connected to the parallel connected DC inputs are thus operated during the operation as a single photovoltaic generator with uniform MPP tracking. Therefore, an operating mode wherein multiple DC inputs of the multi-string inverter are hard-wired is also included in the possible operating modes of a multi-string inverter that can also be used within larger photovoltaic systems.
The hard coupling of different photovoltaic generators can be performed already in the field but also by means of bridging the different inputs of the multi-string inverter and this leads to the corresponding DC/DC converters between the hard-wired inputs on the one hand and the DC intermediate circuit on the input side of the downstream-connected DC/AC converter on the other hand being parallel connected. In this case, even small unbalances of the DC/DC converter can cause the return flow of the direct current from one of the inputs to another of the inputs and as a consequence the cores of the two current-compensated chokes that are allocated to these inputs are already saturated by the unbalanced distributed direct current. These different photovoltaic generators are thus unsuitable for their intended purpose of damping high frequency common-mode interference.
EP 2 276 136 A1 that relates to an overvoltage protection for inverters that have an input-side EMC filter describes an inverter having a DC input stage. The DC input stage includes an EMC filter that has interference suppression inductances in all four supply lines that lead off from two photovoltaic generators, wherein the interference suppression inductances in the two supply lines for each photovoltaic generator are magnetically coupled to a current-compensated choke. Furthermore, the EMC filter comprises interference suppression capacitances that are provided between the respective two supply lines of a photovoltaic generator on the one hand and the supply lines and ground on the other hand. When viewed from the photovoltaic generators upstream of the EMC filter, overvoltage conductors are provided that deflect transient overvoltages to each of the supply lines with respect to one of the other supply lines or ground. When viewed from the photovoltaic generators downstream of the EMC filter, in addition a secondary overvoltage protection is provided that protects the downstream parts of the inverter from transient overvoltages that despite the primary overvoltage protection pass as far as to downstream of the EMC filter or that are even further amplified by means of the EMC filter that is excited in a manner that produces oscillations. The secondary overvoltage protection comprises overvoltage conductors that are connected between dedicated current-carrying supply lines of each of the two photovoltaic generators on the one hand and ground on the other hand and a current-carrying line, combined downstream of the EMC filter, of the two photovoltaic generators on the one hand and ground on the other hand. All current-carrying lines supply current to a DC/DC converter of the DC input stage. Furthermore, buffer capacitances are provided in the DC input stage.
EP 1 209 704 A1 discloses a current-compensated choke having a core embodied from ferromagnetic material for suppressing high frequency interference signals in an electrical circuit of two voltage systems. The electrical circuit comprises at least two switching circuits that have a common reference current path and in each case a dedicated current path. In the two switching circuits, different voltages prevail between the respective dedicated current path and the common reference current path. The two switching circuits have only the reference current path in common. Chokes are arranged in the reference current path and the dedicated current paths of the switching circuits and said chokes are embodied in each case by means of a choke winding of an equal number of windings on a common core. The currents that flow forwards in the individual switching circuits by way of the dedicated current paths and flow back by way of the common reference current path produce mutually compensating magnetizations of the common core. The choke that is multiply current-compensated in this manner and comprises the one choke winding for the common reference current path replaces two current-compensated chokes having in each case two choke windings that would be provided for two completely separate switching circuits. The core of the choke that is multiply current-compensated is preferably an annular-shaped core on which the three choke windings are arranged at a spaced interval of 120°. Based on the figures in EP 1 209 704 A1, it is known to use the known current-compensated choke having three choke windings between a DC/DC converter having multiple outputs and different light sources. A circuit of this type is used in particular in a dual-voltage onboard power supply of a motor vehicle. This is a use in a range of considerably smaller electrical outputs and also smaller currents than flow through an EMC filter in the case of an inverter. Typical electrical lines of a few kilowatts up to a few tens of kilowatts and currents in the range between 10 A and 50 A flow by way of each input of an inverter. In addition, the known circuit does not comprise any interference suppression capacitors that are typical for an EMC filter to ground.
It is known to use a current-compensated choke having a total of three windings on a common core in EMC filters for three-phase alternating currents, wherein each phase of the alternating current is allocated a choke winding.
It is known from US 2010/0207560 A1 in the case of an electric vehicle to guide lines P, C and N between a converter and an inverter through one or more magnetic annular cores. The annular cores are part of an interface between the one converter and the one inverter. Smoothing capacitors are connected between the lines on both sides of the annular core(s). The magnetic annular core(s) cause the effective resonance frequency to be displaced to a frequency at which there is no interference, for example from signal devices. Simultaneously, the annular core(s) increase the inductivity in the case of critical frequencies and consequently reduce the amount of noise that is generated at this frequency.
DE 100 19 461 A1 discloses an interference suppression filter that comprises an interference suppression filter circuit stack that is surrounded at least in part by a magnetic body and forms a choke and capacitor arrangement. Respective mutually insulated interference suppression filter circuits that are stacked one on top of the other are provided in the stack and each of said circuits comprises multiple, mutually insulated LC composite elements that are stacked one on top of the other. These LC composite elements comprise for their part in each case a main coil and a ground coil, both of which are in a spiral shape with an essentially rectangular cross-section and are fastened to one another by means of interpositioning multiple rectangular dielectric discs. In each interference suppression filter circuit, the main coils of the associated LC-composite elements are electrically connected to one another at their inner ends in order to form a main circuit, whereas the ground coils of the LC composite elements are electrically connected at their inner ends in order to form a ground circuit. The interference suppression filter circuits form a filter that comprises a common mode choke coil and a ground capacitor for use as a filter. It is possible to combine three interference suppression filter circuits in order to produce a 3-phase filter. If in the case of the known interference suppression filter, an interference signal is transmitted from the respective main coil to the ground coil, it is possible under certain conditions for a resonance to occur between the inductance and the capacitance and the interference voltage is increased. In order to avoid such resonance occurring, a resistor is provided between the ground coil and a grounding wire. Alternatively, the ground coil itself comprises a certain resistance which eliminates the necessity of a separate resistor.